Extension Line Tool

Extension Line Tool
This tool allows you to draw extension or witness lines from any
line or flat side of an object in the drawing.

To draw a witness line from a line or flat side of an object,
select the Extension Line tool from the Dimension Tool palette.

Place the cursor in the drawing area over a line or flat side
of a rectangle or polygon. Press the mouse button. Perpendicular
snaps are available on request prior to clicking the mouse.

Drag in the direction that you want the witness line to go.
As you 'pull' the line out, you will notice that there is a witness
gap between the object and the witness line. Press the Shift key
to eliminate the witness gap. Release the mouse button to end
the line.

The witness line will be placed in the drawing.

To draw an extension line from a dimension line:

Place the cursor in the drawing area over a dimension line.
(In this case, you've already dragged the dimension text to one
side.) Press the mouse button.

Drag in the direction that you want the extension line to go.
Because you have clicked on a dimension line, there will be no
witness gap; however, you may press the Shift key to create a
witness gap. Release the mouse button to end the line.